The present invention relates to optical inspection of workpieces and, in particular, to a method for inspecting etched semiconductor workpieces using the principal components of scattered light intensity to distinguish differences in the nature of etched lines, including differences in profile, depth, slope and linewidth.
It is common and necessary practice in the manufacture of integrated circuits to periodically inspect etched wafer workpieces for changes in critical line width, line profile and etched depths. Previously such inspections could be performed rapidly and non-destructively by a technician using an optical microscope. But as semiconductor technology has evolved to micron and sub-micron line widths, inspection by optical microscopy is no longer adequate. With today's submicron patterns, inspection typically involves selecting a sample of wafers from a processing lot, cleaving the wafers and inspecting the cleaved wafers with a scanning electron microscope. This approach is time-consuming, destructive and limited to a small subset of the wafer lot.
In an effort to replace this time-consuming and destructive testing process, a variety of experimental efforts have been directed toward the development of inspection and monitoring methods based upon the scattering of a beam of light directed onto the workpiece surface. Such techniques have typically utilized only a narrow range of scattering angles, typically the specular intensity and the first order. While promising experimental results have been reported, these techniques have not demonstrated the ability to discriminate between submicron variations in etching depth and variations in etching profile.